Workshop Safety and Uncertainty

Uncertainties have a significant influence on the assessment and evaluation of the safety of a repository system for high-level radioactive waste. Significant reasons for uncertainties concerning the safety barriers of a repository are: Conducting experiments on the long-term behaviour of the repository in real time is impossible due to the long assessment period over which the repository is supposed to ensure safety. The extrapolation of results from time-limited experiments, e.g. on the corrosion of container materials, to other temporal dimensions is associated with uncertainties. Uncertainties also stem from differences between experimental situations, e.g. laboratory experiments, and the real conditions in the repository. The interpretation of empirical results can be ambiguous and therefore associated with uncertainties. The development of future impacts on the barriers can only be predicted to a limited extent. Therefore, the future behaviour of the barriers can only be extrapolated into the future to a limited extent on the basis of experience gained in the past and uncertainties remain. The construction and operation of the repository will disturb its natural environment. The geological environment in which the repository is embedded behaves differently from a natural geological system, which in turn is associated with uncertainties. A major source of uncertainties is also the natural inhomogeneity of the geological barrier, which can only be investigated on a sample basis. During excavation and other construction work underground, unforeseen situations are to be expected, which make it necessary to act situationally. The complexity of the disposal path where decisions are interlinked, creates further uncertainties. Last but not least, it is uncertain what further findings on the safety of the repository will be obtained in the future along the disposal path. For safety studies, especially studies on the long-term safety of repository systems, methods and conventions for dealing with uncertainties have become established internationally. In the site selection process, these methods and conventions are questioned and, if necessary, must be further developed so that they ultimately also convince the interested public and scientists from other disciplines. In the workshop, uncertainties will be examined in particular from the perspectives of a civil engineer and of a materials researcher with introductory presentations. This will be followed by a moderated discussion. The workshop will focus on the preliminary safety investigations; however, the discussion can also refer to later phases of the disposal path. The aim of the discussion is to arrive at a common synthesis: Where have good practices for dealing with uncertainties already been established? Where is there still a need for research and clarification? What needs to be considered in the dialogue with the interested public?

Suitability of flat bedded salt formations in Germany as the site for a repository for heat-producing radioactive waste

The search for a site of a final repository for highly radioactive waste in Germany was renewed when the Site Selection Act (StandAG) came into force in 2013. In Germany the development of concepts for a final repository and safety analyses for a repository in a salt dome was prioritized for many decades, whereas repository concepts in clay and crystalline rock were first considered only two decades ago. The aim of a comparative site selection procedure is to find a site before 2031, which provides the best possible safety for the enclosure of highly radioactive waste over a time period of 1 million years. The fundamental safety-related consideration is the enclosure of toxic waste in a so-called containment effective geological area (ewG). The main aspect of a long-term proof of safety is logically the systematic proof of safe long-term enclosure of the deposited waste. The approach developed within the framework of appropriate research projects (e.g. Eickemeier et al., 2013) is essentially based on the proof of geotechnical integrity of the ewG as the fundamental geological barrier as well as the geotechnical barriers. Due to their unique characteristics, including imperviousness and plastic deformability, salt rocks have been used for decades in Germany and worldwide in mining and especially for energy storage. Whereas halite in salt domes (type steep​​​​​​​ inclined salt) is distributed particularly in northwest Germany, flatly deposited salt rock (type flat bedded salt​​​​​​​) dominates in middle Germany and salt pillows (type salt pillows) in parts of northeast Germany. Both types of “bedded salt”​​​​​​​ widely reflect in their lateral extension sedimentation-related deposition conditions, apart from diagenetically related alterations. Beginning with the presentation of the host rock-specific boundary conditions of the various rock salts, this article focuses on the appropriate procedures for the proof of integrity of the geological barrier rock salt, based on the available experiences, corresponding reference studies and analogous examples. In the results it is shown that repository concepts in bedded salt formations and especially in the constellation of salt pillows provide substantial safety-related advantages due to a site-specific multibarrier system with alternate deposition of salt and saliferous clay as well as an intact overlying rock covering.

Discretization of the Water Uptake Process of Na-Montmorillonite Undergoing Atmospheric Stress: XRD Modeling Approach

This work focuses on the water-montmorillonite interaction under variable atmospheric conditions in order to characterize the interlamellar space (IS) configuration for possible use in the context of geological barrier for radioactive and industrial waste confinement. Atmospheric stress is achieved by applying, for Na-rich montmorillonite, a water sorption/desorption constraint, created at the laboratory scale. This hydrological disturbance allows the “demolition” of the clay history and to highlight the clay hydrous performance. The structural analysis is achieved using modeling of XRD profiles, which allowed us to determine the optimal structural parameters describing the IS configuration along thec∗axis. During the “in situ” XRD analysis, a sorption/desorption cycle is envisaged by variation of the relative humidity rate (%RH) from the saturated condition (94 %RH) towards extremely dry ones (2-3 %RH). Qualitatively, a new hydration hysteresis behavior of the “stressed” sample appears. Structural analysis achieved before and after perturbation allowed us to identify, respectively, the homogeneous hydration states, the hydrous transition domains, and the hydration heterogeneity degree. This latter parameter is characterized, quantitatively, by variable relative abundances of mixed-layer structure (MLS) population discerned over a wide explored RH range. Using the optimum structural parameter, the water molecule distribution versus the applied hydrous strain was quantified.

Quantification of the Natural Factors’ Impact Effectiveness on Environmental Hazards – Slope Movements in the Flysch Areas of the Kysuce Region

The flysch areas belong to the territories with highest occurrence of landslides in Slovakia. Almost 67% of all landslides in Slovakia take place within the Carpathian flysch. It is a type of slope deformation that responds sensitively to the quality of individual factors that form the landscape and to the change in natural conditions. The occurrence of various geodynamic phenomena can be understood as a geological barrier that reduces or inhibits the use of natural environment and negatively affects the life of society and territorial development. In this paper, we evaluate the statistical significance of selected natural factors of the landscape in relation to the occurrence of unstable slopes in the Kysuce region. In addition, we also evaluated the expansion of unstable slopes in individual landscape factors. Significant linkages between the variables’ hydrogeological base_sandstones with thin clay layers (HB_s) and geological-substrate complex_loamy wastes to loamy-stony debris on sandy conglomerates (GSC_sc) (R = 0.95, p = 0.002) and secondary significant linkages between the variables soil type: Dystric Cambisols (S_CMd) and HB_s (R = 0.40, p = 0.002) (Klokočov and Zákopčie cadastres) were observed. Significant correlation of variables within the areas with unstable slopes was also observed between hydrogeological base_sandy flysch (HB_sf) and geological-substrate complex_loamy wastes on flysch stones (GSC_fs) (R = 0.81, p = 0.002) (Nová Bystrica and Kysucké Nové Mesto cadastres). The most unstable slopes occur in Nová Bystrica cadastre (34.62% of the area) and in the Klokočov cadastre (28.25% of the area). The inclination of slopes (especially slopes above 12°) plays an important role within the unstable slopes. Slopes with steep inclination cover up to 81.45% of the cadastral area of Nová Bystrica, of which almost 1/3 are unstable slopes.

Effect of mechanical constraint on the hydration properties of Na-montmorillonite: study under extreme relative humidity conditions

The evaluation of the performance of a geological barrier, consisting essentially of a clay matrix, in the context of industrial and household waste confinement must go with the study of its hydration behavior respectively under extreme atmospheric conditions and variable mechanical soil constraints. Na-montmorillonite (Swy-2) is used, as starting materials, in order to establish the link between applied externals strain (variable relative humidity rate %RH and axial mechanical constraint) and the hydration material response. All constraints are realized at the laboratory scale. This work is achieved using oedometric testing and quantitative X-ray diffraction (XRD) analysis, based on the modeling approach, which consists in the comparison of experimental 00l reflections with the calculated ones deduced from structural models. This approach allows us to quantify the interlamellar space configuration and all structural changes along the c* axis. Obtained results show a decrease for the void ratio e value along the compaction/reswelling process. The “insitu” XRD analysis realized at 5%RH demonstrates hydration shift, from dehydrated water layer (i.e. 0W) to monohydrated water layer (i.e. 1W), attributed to the applied mechanical constraint. At 90%RH, the sample hydration state remains at tri-hydrated water layer (3W) with a clear interstratified trends.

Poromechanical behaviour of a surficial geological barrier during fluid injection into an underlying poroelastic storage formation

A competent low permeability and chemically inert geological barrier is an essential component of any strategy for the deep geological disposal of fluidized hazardous material and greenhouse gases. While the processes of injection are important to the assessment of the sequestration potential of the storage formation, the performance of the caprock is important to the containment potential, which can be compromised by the development of cracks and other defects that might be activated during and after injection. This paper presents a mathematical modelling approach that can be used to assess the state of stress in a surficial caprock during injection of a fluid to the interior of a poroelastic storage formation. Important information related to time-dependent evolution of the stress state and displacements of the surficial caprock with injection rates, and the stress state in the storage formation can be obtained from the theoretical developments. Most importantly, numerical results illustrate the influence of poromechanics on the development of adverse stress states in the geological barrier. The results obtained from the mathematical analysis illustrate that the surface heave increases as the hydraulic conductivity of the caprock decreases, whereas the surface heave decreases as the shear modulus of the caprock increases. The results also illustrate the influence of poromechanics on the development of adverse stress states in the caprock.

Multibarrier system preventing migration of radionuclides from radioactive waste repository

Safety of radioactive waste repositories operation is associated with a multibarrier system designed and constructed to isolate and contain the waste from the biosphere. Each of radioactive waste repositories is equipped with system of barriers, which reduces the possibility of release of radionuclides from the storage site. Safety systems may differ from each other depending on the type of repository. They consist of the natural geological barrier provided by host rocks of the repository and its surroundings, and an engineered barrier system (EBS). The EBS may itself comprise a variety of sub-systems or components, such as waste forms, canisters, buffers, backfills, seals and plugs. The EBS plays a major role in providing the required disposal system performance. It is assumed that the metal canisters and system of barriers adequately isolate waste from the biosphere. The evaluation of the multibarrier system is carried out after detailed tests to determine its parameters, and after analysis including mathematical modeling of migration of contaminants. To provide an assurance of safety of radioactive waste repository multibarrier system, detailed long term safety assessments are developed. Usually they comprise modeling of EBS stability, corrosion rate and radionuclide migration in near field in geosphere and biosphere. The principal goal of radionuclide migration modeling is assessment of the radionuclides release paths and rate from the repository, radionuclides concentration in geosphere in time and human exposure to ionizing radiation